General Concepts

General Concepts
Energy provided by the fan creates a motive force,or pressure,divided into two components: static pressure and dynamic pressure as defined below. a) Static pressure, Ps, is the result of compressing fluid (air) within a duct. It is measured with reference to atmospheric pressure.Static pressure reaches a peak atthe fan unitand decreases throughoutthe ductdue to frictional pressure losses and declines to almostzero atthe exit.The same occurs in the exhaust duct, although in this case the value is negative. It is ‘positive’ during suction and ‘negative’during ‘discharge’.
b)Dynamic pressure, Pd, is the energy component due to fluid velocity and is calculated using following formula:
Where: r=airflow density (kg/m2) v=airflow velocity (m/s)
Dynamic pressure is always positive.The velocity varies with changes in duct geometry, size etc along the ductlength,as the air mass atany pointin time is the same throughoutthe duct.This is the case until its exitpointor when air is distributed into various branches of the ductnetwork.
c) Total pressure, Pt, is the algebraic sum of Ps + Pd.P t is positive in supply duct and negative in the discharge duct.
Units and measuring equipments
The international unitof pressure is the Pascal (1Pa = 1N/m2). However,calculations relating to pressure in HVAC systems is conventionally expressed in mm of manometer hydrostatic head.The conversion factor is 1 mmwg= 9.81 Pa (‘mmwg’,or sometimes expressed ‘mmca’,is the measurementin millimetres of water measured in the manometer).
The instrument for such measurements is the Pitot-Static Tube,illustrated in the adjacentfigure.
6.2Pressure losses
The movementof the air (akin to the movementof a fluid) inside ducts causes two types of pressure loss:friction losses and dynamic losses.
a) Pressure losses by friction
Frictional losses are influenced by the viscosity of a fluid (in this case,air),changes in the direction of the air and the behaviour of air molecules as part of the turbulent effect;‘normal’operating conditions in HVAC systems.
Losses take place along the length of the ductand are expressed in Pa/m or mmwg/m (total pressure by the length of the duct).
The formulaic calculation ofthe pressurelosses is complex,since itdepends on a considerable number of factors including exponential equations,established by Darcy-Weisbach and Colebrook.These formulae can be calculated with computing tools and the appropriate software.
If no software is available,a more convenient method is to use friction graphs already created to describe a duct’s geometry. Material type (using only the friction coefficient), air conditions of density and temperature,as well as the atmospheric pressure are also taken into account.
If considering another type of installation,corrective factors have to be applied to the data from the graph,which provide values for the real pressure losses of the system.
Pressurelosses in ISOVER’sglass wool ductboards
Laboratory investigations and practical experience of duct assemblies with diverse cross-section sizes and types haveestablished the following: •Real pressure losses are practically equal to the theoretical values predicted by ASHRAE’s friction graphs for cylindrical galvanized metal ducts,for air speeds from 0to15m/s. •Elbows with two 135º-angles,thatis to say,those made from straightductsections,have similar or slightly inferior pressure losses compared to curved elbows made of glass wool ductboards